1. Field of the Invention
The present invention relates to the shape of a dot mark and a method of forming the dot mark which is formed for product management or security in a specified position on the surface and the like of an item to be marked such as a semiconductor wafer, in a minute area on a scribe line, a rear surface of a wafer, a peripheral surface of a wafer, or an inner surface of a V-notch, a glass substrate such as a liquid crystal substrate, an electrode (pad) such as a bare chip, the surface of an IC, the rear surface of an IC, various ceramic products, and a lead portion of an IC. More specifically, the present invention relates to the shape of a dot mark of a small size having particular shape which secures optical visibility, and a method of forming the dot mark.
2. Description of the Related Art
For example, in semiconductor manufacturing processes, it is necessary to set various and strict manufacturing parameters for each process. In order to control the parameters, a mark such as numeral, character, or bar code is displayed by dots on the surface of a part of a semiconductor wafer. The number of manufacturing processes of a semiconductor is 100 or more and, moreover, a number of device forming processes and planarization processes are performed in each process. The processes include, for example, resist application, projection of a pattern onto a resist in a reducing manner, resist development, and planarization of various films such as an insulating film and a metal film for filling a gap which occurs by copper wiring or the like.
On the other hand, the mark made by dots is generally formed by irradiating the surface of a part of a semiconductor wafer with a continuous pulse laser beam through an optical system. The marking is not limited to once. In order to show historical characteristics of manufacturing processes, minimum historical data required in the manufacturing processes is usually marked. Since the marking area on the semiconductor wafer is, however, limited to an extremely narrow region, the size of the dot and the number of dots to be marked are accordingly limited. The marking area, the size of a dot, and the number of dots are specified by the SEMI standard and the like.
As disclosed in, for example, Japanese Laid-Open Patent Publication No. 2-299216, information of a semiconductor wafer on which the dot marking is performed is read as a change in reflectance of an emitted laser beam of an He-Ne laser or a change in vibration of a heat wave of an ordinary laser beam. On the basis of the read information, various manufacturing parameters in subsequent manufacturing processes are set. When the information is not accurately read and is read erroneously, therefore, all of semiconductor wafers become defective except for a coincidence. Most of the causes of the defective reading relates to an unclear mark formed by the dot marking. One of factors of the unclearness is the shape of a dot as an element of the mark.
It is generally considered that the influence by the depth of a dot is large. As disclosed in, for example, Japanese Laid-Open Patent Publication No. 60-37716, a dot is usually formed by melting and removing a part of the semiconductor wafer in a spot state with irradiation of a laser beam of a large energy so as to obtain a required dot depth. In, this case, the melted and removed portion is piled around the dot or scattered and adhered to the peripheral portion of the dot, and it may prevent device formation and exerts a large influence on the quality. Further, in case of dot marking performed by a YAG laser, due to the particularity of the YAG laser or the Q switch operation, a fluctuation is apt to occur in a laser output and a variation occurs in the depth or the size of a dot.
In order to solve the problems, for example, as disclosed in Japanese Laid-Open Patent Publication No. 59-84515 and 2-205281, the same point is repeatedly irradiated with a pulse laser beam of a relatively small energy. In the former Publication, in order to form a dot mark, while sequentially reducing the dot diameter pulse by pulse in order, the same point is repeatedly irradiated with a laser beam, thereby forming a deep dot. In the latter Publication, the frequency of a laser pulse of the first time is set to 1 kHz or lower and the frequency of a laser pulse subsequently emitted is set to a high repetitive frequency of 2 to 5 kHz to thereby form a dot having a depth of 0.5 to 1.0 xcexcm or 1.0 to 1.5 xcexcm.
On the other hand, since generation of dusts can not be avoided by the marking method as described above, a laser marking method which provides excellent visibility and suppresses the generation of dusts has been proposed in, for example, Japanese Laid-Open Patent Publication No. 10-4040. The disclosure of the publication relates to a laser marking method of forming a dot mark by projecting a liquid crystal mask pattern onto the surface of a semiconductor material by emitting a pulse laser beam, in which the energy density is set to 18 to 40 J/cm2, the pulse width is selected within a range from0.05 to 0.40 ms, the surface of the semiconductor material is irradiated with a pulse laser beam, and a number of small protrusions are created in the laser irradiation region in a process of melting and recrystallizing the surface of the semiconductor material.
According to the marking method, by the irradiation of the laser beam which is emitted on a pixel unit basis, a number of small protrusions each having the height of about 1 xcexcm or less and the diameter of 0.5 to 1. 0 xcexcm are formed on the surface of an article to be marked. The interval between neighboring protrusions is about 1.5 to 2.5 xcexcm and the density of the protrusions is 1.6 to 4.5xc3x97107 pieces/cm2. The aggregation of a number of small protrusions are handled as a single dot mark which is read by utilizing an irregular reflection of light, and in case of such small protrusions, the generation of dusts in the event of the formation can be suppressed.
It is certain that one of the causes of unclearness of the dot mark having a hole shape (hereinbelow, the clearness of the dot is called xe2x80x9cvisibilityxe2x80x9d) relates to the depth of the dot. Even when the dot is formed deep enough, however, in the case where the diameter of the opening is large, for example, when a laser beam strong enough to obtain a required depth is emitted, the energy density has generally a Gaussian distribution. The dot has therefore a smooth curved surface which is a gentle slope as a whole, so that a case where the difference between the dot and the peripheral area is not easily discriminated by the reading means as described above occurs. In the above publication of Japanese Laid-Open Patent Publication No. 2-205281, although the dot depth is specifically described as 0.5 to 1.0 xcexcm or 1.0 to 1.5 xcexcm, the diameter of the dot is not described at all, and the shape of the dot is merely described as a Gaussian shape.
In the disclosure of Japanese Laid-Open Patent Publication No. 59-84515, since it is described that the diameter of the dot opening of the first time is 100 to 200 xcexcm and the depth is 1 xcexcm or less and, specifically, the laser beam is emitted four times, the depth of the dot in this case is at most 3 to 4 xcexcm. In the drawings of the publication, the shape of the dot formed at a time is similar to the Gaussian shape.
It can be therefore considered that dots each having a required depth, whose sizes are uniform to a certain degree are formed by any of the marking methods disclosed in the above publications. The shape of the formed dot is, however, like a conventional shape and the diameter with respect to the depth is extremely large. Thus the visibility still lacks certainty. Since the reduction in size (diameter) of the dot to be formed is not disclosed, the disclosure does not intended to reduce the conventional dimension of 50 to 150 xcexcm. The numerical values at the present time point which are specified by, for instance, the SEMI standard are simply used. A substantial large increase in the number of dots and the dot forming area cannot be therefore expected and, moreover, it is difficult to mark various information.
The visibility of the dot mark is high when there is a large difference with respect to the light reflecting direction and the reflection amount between the mark and the periphery. When the depth is relatively large with respect to the diameter of the opening, the visibility is therefore high in the following manner. Since the reflecting direction of reflection light in the hole, incident at a predetermined incident angle, is not regular but is irregular, the reflection light outgoing from the opening of the hole to the outside is reduced. Assuming that the peripheral area of the hole is a smooth surface, the reflection light in the peripheral area is reflected in the same direction, so that the lightness is high. The visibility is high when the difference between the lightness and darkness is large.
The small protrusions formed by the marking method disclosed in Japanese Laid-Open Patent Publication No. 10-4040 are too small to be observed individually. The difference between the irregular reflection light amount of the irregular reflection surface as a surface of collection of the protrusions and the reflection light amount of the smooth surface is small, so that it is difficult to distinguish the irregular reflection surface from the peripheral smooth surface. The visibility is inevitably not good. Further, according to this publication, when an irradiated energy density is less than 18 J/cm2, the small protrusion is not formed because the surface is not molten, however, this is because the pulse width is quite large and no special arrangement is made in the marking apparatus used.
Since a single dot mark is composed of a collection of small protrusions and there is no description regarding the size of one dot mark, it is regarded that the size of the dot is the same as that of the conventional dot and the dot mark forming area is limited. Even if the size of the dot which is the aggregation of the small protrusions obtained is small, the shape and size of a plurality of extremely small protrusions which are dispersed in one dot can not be controlled to be uniform, which makes no difference in lightness from its periphery so that the visibility of each dot further deteriorates.
The invention has been achieved to solve the above-described problems. Specifically, a first object is to obtain a dot mark shape having small and uniform shape and size and excellent visibility can be realized even by a single dot mark and a second object is to provide a dot marking method for accurately forming such a microdot mark. The other objects will become clear from the following description.
The objects are effectively achieved by the present invention.
The inventors of the present invention newly, specifically examined and analyzed conventional dot marking apparatuses and methods of such kinds and the dot shapes formed and found that the main factor which makes a microdot certainly visible though it is small is the dot shape and the ideal shape cannot be obtained by the conventional marking apparatuses and methods.
To be specific, for example, as shown in FIG. 2 and disclosed in Japanese Laid-Open Patent Publication No. 2-205281, according to the conventional marking apparatus, first, a character to be printed on a semiconductor wafer and a marking mode is set by an input unit 21. A marker controller 22 controls an ultrasonic Q-switched element 23, an internal shutter 24, an external shutter 25, an attenuator (optical attenuator) 26, and a galvanometer mirror 27 in order to mark a dot having a predetermined depth onto a wafer W in accordance with the set marking mode and one dot is marked by one Q-switched pulse. In FIG. 2, reference numeral 11 denotes a total reflection mirror; 12 an internal aperture (mode selector); 13 a lamp house; 14 an output mirror; 15 an aperture: 16 a leveling mirror; 17 a Galilean expander: 18 an aperture; 19 an f-xcex8 lens; and 20 a YAG laser.
According to such a general marking method, as described above, since the energy density distribution of the laser beam emitted to the surface of the semiconductor wafer has the Gaussian shape, the inner surface of the dot formed on the surface of the wafer is gently curved by the influence of the energy density distribution. The marking methods are based on the invention of U.S. Pat. No. 4,522,656. The invention of this U.S. Patent is characterized in that, by irradiating the surface of a wafer with a laser beam having the diameter which is 1.5 to 6.5 times as large as the diameter of a dot to be marked, thermal conduction to the peripheral area is prevented, the energy is effectively used, and the central portion of the irradiation point is melted to form a hole.
In other words, the method effectively uses the energy density distributed in the Gaussian shape of the laser beam. The energy in a part corresponding to the bottom of the energy density distribution, in which the laser intensity is low, is directed to the periphery of the hole opening portion to thereby warm the periphery, prevent loss of the thermal energy by the heat conduction from the central portion of the hole, and effectively realize the hole formation in the central portion. However, since a part of the laser energy is not directly used for the hole formation but is consumed, the efficiency is low. Moreover, the heat history remains in the hole peripheral portion by irradiating the periphery of the hole with a laser beam and an adverse influence may be exerted on the product. Further, the marking method can form only a shallow dot mark having a large dot diameter as described above and the peripheral portion of the hole is swollen. It further deteriorates the visibility.
The inventors further examined the shape of the dot mark having excellent visibility and, as a result, found that by setting each of the pulse width and the energy density of the laser beam within a predetermined range as will be described hereinlater and controlling the energy density distribution, a dot mark formed by each laser beam emitted to the surface of an article to be marked has a peculiar shape which is not conventionally known and, though it is a single microdot mark, it has visibility higher than that of a dot mark shape having a recess formed by the conventional laser marking.
That is, according to the first aspect of the invention, there is provided a dot mark formed on the surface of an article to be marked by using a laser beam as an energy source. Although it is a microdot mark having a length along the surface of the article to be marked of 1.0 to 15.0 xcexcm, the dot mark has the shape which is very visible and is made of dots each formed by a laser radiation point. The central portion of each dot mark has a protrusion which protrudes upward from the surface of the article to be marked and the height of the protrusion is 0.01 to 5 xcexcm. In view of visibility, it has been found that the dot mark of the invention has the protruding shape, the height in the above mentioned range is sufficient because the dispersing light rather than regular reflection light is detected.
In order to clarify the mechanism of forming such a dot mark shape, the inventors have carried out a number of experiments from various viewpoints and, as a result made the inference as described below. Needless to say, other various inferences can be drawn.
That is, when each of dot forming positions is irradiated with a laser beam, the surface of the irradiated portion of the article to be marked is melted and a pool of the melted material (hereinbelow, called a melted pool) is created. At this time, the temperature of the melted material becomes lower toward the bank of the melted pool and becomes higher toward the center. Due to the temperature gradient, a distribution occurs in the surface tension, and movement occurs in the melted material. Simultaneous with the stop of the pulse irradiation, cooling starts and the material is solidified. In a state where the material is melted, the central portion of the melted pool is a free interface and the bank of the melted pool corresponds to a fixed end, so that the state is similar to that of a film whose periphery is fixed. In such a state, the surface tension acts and a dynamic motion similar to a film vibration occurs in the central portion of the melted pool.
The length of an amplitude in the film vibrating mode is determined substantially by the viscosity peculiar to the material and the surface tension, Therefore, the larger the diameter of the melted pool becomes, the number of vibrations increases. For example, in case of silicon, since the length of an amplitude is about 3 to 5 xcexcm, a microdot shape having an effective contrast can be obtained in a small area. It was also confirmed by the experiments that a dot mark can be formed in the small area with little influence of the gravity.
When the laser irradiation pattern is square, the melted pool is accordingly square. When it is circular, the melted pool is accordingly circular. Further, the vibration similar to that of a film also occurs in a mode corresponding to the square or circular shape. FIGS. 22 to 31 schematically show square and circular film vibration modes. As the vibration mode becomes higher, the number of vibration waves increases and the vibration mode is shifted between a recessed pattern and a protruded pattern. It can be understood also by experimental results which will be described hereinlater that the motion of the melted pool has a strong correlation with the film vibration.
FIG. 22 shows a circular film vibration mode in a state where the surface of an article to be marked is expanded as an upward curved surface. FIG. 23 shows a circular film vibration mode in a state where the surface of an article to be marked is contrarily recessed as a downward curved surface. FIG. 24 illustrates a circular film vibration mode in a state where a ring-shaped recess is formed and the surface protrudes upward in an almost conical shape in the center of the ring-shaped recess on the surface of an article to be Marked. FIG. 25 shows a circular film vibration mode having a ring-shaped expanded portion and recessed a downward curved surface in the center of the expanded portion. FIG. 26 shows a circular film vibration mode having a ring-shaped expanded portion and conically protruding upward in the center of the expanded portion. FIG. 27 shows a circular film vibration mode concentrically having a ring-shaped recessed portion as an outermost portion, an expanded portion, and a recessed portion on the surface of an article to be marked.
FIGS. 28 to 31 show square-shaped film vibration modes corresponding to FIGS. 22 to 25, respectively. FIG. 31 in this case is peculiar in that an expanded portion does not have a simple ring shape but a waved shape in which the corners of a square are largely expanded.
As a result of a number of experiments, it was found that the dot mark shape in any of the film vibration modes is incomparably smaller than the conventional one and can be obtained by setting the pulse width and the energy density of the laser beam as marking parameters within their predetermined ranges and controlling the energy density distribution.
The laser marking apparatus disclosed in Japanese Patent Application No. 9-323080 proposed before by the inventors is a preferred example of a laser marking apparatus used to form the dot mark shape according to the first aspect of the invention. Since the detailed construction is described in the specification of the application, a simple description will be given here.
Reference numeral 1 in FIG. 1 denotes a marking apparatus for marking characters, bar code, 2D code, or the like on the surface of an article to be marked by using a laser as a light source. The marking apparatus 1 comprises a laser 2, a beam homogenizer 3 for homogenizing the energy distribution of a laser beam emitted from the laser 2, a liquid crystal mask 4 which is arranged to transmit/absorb the laser beam in accordance with the display of a pattern, beam profile converting means 5 for converting the energy density distribution of the laser beam corresponding to each pixel in the liquid crystal mask 4 into a required distribution, and a lens unit 6 for condensing the beam passed through the liquid crystal mask 4 onto the surface of the semiconductor wafer on a dot unit basis. The maximum length of one dot in the liquid crystal mask 4 is 50 to 2,000 xcexcm and the maximum length of one dot in the lens unit 6 is 1 to 15 xcexcm.
In order to form a microdot having such a shape, it is necessary to very accurately control the quality and quantity of the laser beam irradiated on a dot unit basis. In order to obtain a laser beam having a very small diameter of the invention from a laser beam having a large beam diameter, a high-quality high-power laser beam is necessary. It is however difficult to condense the laser beam any more due to the diffracting phenomenon of the high-power laser. Even if the laser beam can be condensed more, the angle of outgoing radiation of the lens becomes large and the depth of focus becomes extremely short, so that it is difficult to consider that an actual process can be performed. Further, an ultraprecise lens system is also required from the view points of resolution and the like. In case of equipping such a lens system, the cost of the facilities increases more, so that the lens system cannot be applied from the economic point of view.
In order to realize a microdot mark with an ordinary lens system, the laser beam itself emitted from the laser 2 is split and converted into laser beams each of a smaller diameter having an energy necessary and sufficient to mark one dot, and the energy density distribution of the laser beam on the dot unit basis has to be converted into a profile suitable to form the dot shape. In order to form a suitable and balanced profile, it is necessary to homogenize the energy density distribution of the laser beam on the dot unit basis, which is not yet converted, prior to the forming.
In order to obtain the light source for the microdot mark, It is rational to use the liquid crystal mask 4 in which liquid crystals of the liquid crystal mask 4 each capable of arbitrarily transmitting/absorbing light in accordance with various data written in the central control unit are arranged in a matrix.
It is important to convert the laser beam emitted from the laser having the Gaussian energy density distribution firstly into that having a homogenized shape similar to, for example, a top-hat shape by using the beam homogenizer 3. The types of the beam homogenizer 3 are as follows; a type of irradiating the surface of the mask at once by using, for example, fly""s eye lens, binary optics, or cylindrical lens, and a type of irradiating the surface of the mask with a beam by using an actuator such as a polygonal mirror or a mirror scanner.
When the laser beam whose energy density distribution has been homogenized by the beam homogenizer 3 has then to be converted again into a profile of an energy density distribution suitable to obtain the preferred dot shape, the beam profile converter 5 is further used. As the beam profile converter 5, for example, a diffraction optical element, a holographic optical element, an opening mask or a liquid crystal mask having absorption/transmission regions, a convex or concave microlens array, and the like can be used. The beam profile converting means is not always necessary to obtain the dot mark shape of the invention.
The article W to be marked as a processing target in the invention is a semiconductor water, a glass substrate such an a liquid crystal substrate, an electrode (pad) such as a bare chip, the surface of an IC, various ceramic products, a lead portion of an IC, or the like. The semiconductor wafer is represented by a silicon wafer itself. It also includes a wafer on which an oxide film (SiO2) or a nitride film (SiN) is formed, an epitaxial wafer, and a wafer on which gallium arsenide or indium phosphorus compound is formed.
The second aspect of the invention provides a marking position particularly preferable for the dot mark having particular shape. Namely, the surface on the article to be marked by dot marking is specified to be a beveled portion on an outer periphery of the wafer. It has been proposed conventionally to provide marking on the outer periphery of the wafer, however, the mark is consisted of so-called bar code. And when a usual dot mark is required to be formed on said surface, it is difficult to be formed on a small area due to its large size. Even when it is small, optical reading of the regular reflection light has been difficult. However, the dot mark of the invention is small and has a particular shape, and thus it has been found that sufficient optical visibility can be secured by utilizing the dispersing light from the surface of the protrusion. Further, when the dot mark is formed on the beveled portion of the wafer as described above, the mark would be hardly lost due to various treatments in the process of finishing the wafer.
The third aspect of the invention provides a marking method suitable to form the microdot mark having a peculiar shape on the surface of an article W to be marked. Even when the marking apparatus 1 is used, as long as the marking parameters specified by the third aspect of the invention are not satisfied, the dot mark of the invention having the above-mentioned peculiar shape cannot be obtained.
Specifically, the method according to the third aspect of the invention comprises the steps of: homogenizing an energy distribution of the laser beam emitted from the laser 2 by the beam homogenizer 3 as described above; forming a desired pattern by controlling the liquid crystal mask 4 in which the maximum length of each pixel is 50 to 2,000 xcexcm and irradiating the liquid crystal mask with the laser beam homogenized by the beam homogenizer 3; setting the energy density of the laser beam on the dot marking surface, which passed through the liquid crystal mask 4 to 1.0 to 15.0 J/cm2; and condensing the laser beam for each dot by the lens unit 6, which passed through the liquid crystal mask, onto the surface of the article to be marked so that the maximum length of each dot is set to 1.0 to 15.0 xcexcm.
In order to form the dot mark having the peculiar shape of the invention, the inventors of the present invention have carried out a number of experiments to know how the wavelength, the energy density, and the pulse width of a laser beam exert an influence. As a result, the wavelength makes only the absorption ratio of the semiconductor wafer different but does not change others. When silicon is used as a material of the semiconductor wafer as an example, in order to obtain the dot mark shape of the invention, It Is necessary to moderately reduce the penetration to silicon as the dot shape becomes smaller. Consequently, the most preferable result is obtained at about 532 nm. Though the wavelength cannot be unconditionally specified since it differs according to the material of the article to be marked, preferably it is within the visible range of 300 nm to 700 nm.
On the other hand, with respect to the pulse width, a range in which a permissible range of the energy density can be set properly wide and the output of the laser can be suppressed as much as possible was examined. As a result, the range of 10 to 500 ns was found to be effective to form the dot mark of the invention. More preferably, the range is 50 to 120 ns. In the case of 500 ns or larger, the energy density becomes too high, so that the desired dot mark shape is not easily obtained and the laser itself inevitably becomes large. It should be understood that these values are quite small comparing with the pulse width according to the marking method disclosed in the above mentioned Japanese Patent Publication No. 10-4040. In a process performed to a ps area with a laser, transpiration occurs considerably and the permissible energy density range is extremely narrow.
The energy density largely depends on the laser wavelength, pulse width, and the optical characteristics of a material to be processed. Consequently, it is preferable to determine the energy density in consideration of both of the laser wavelength and the pulse width. In case of previously specifying the values of the laser wavelength and the pulse width as described above, it is appropriate to set the energy density of the laser beam on the dot marking surface, which passed through the liquid crystal mask 4 and split, to 1.0 to 15.0 J/cm2. Assuming that the wavelength is the same, within the above mentioned pulse width range, when the pulse width is the smaller one, it is preferable that the energy density is within the range of 1.0 to 3.7 J/cm2, and when the pulse width is the larger one, it is preferable that the energy density is within the range of 3.7 to 11.0 J/cm2.
Strictly saying, a very thin native oxide film is formed on the surface of a semiconductor wafer, especially, made of silicon. In the invention, the oxide film is simultaneously deformed. It is therefore necessary to take the following points into account in order to preferably deform the oxide film.
1) The melting point of the oxide film (SiO2) is higher than that of a silicon wafer (Si).
2) The oxide film is amorphous and has no clear change point to a liquid phase. It is softened around the melting point of silicon.
3) The oxide film is transparent from a visible region to a near infrared region and absorbs silicon.
From the above points, at the event of pulse irradiation, the silicon wafer is directly heated and melted through the oxide film. The oxide film is softened by thermal conduction from silicon and is formed in dots in accordance with the surface shape of silicon by elastic deformation. When the oxide film becomes thicker, however, the temperature rise in the oxide film by the thermal conduction is not sufficient at the interface of the oxide film, which is in contact with the outside. As a result, the temperature rise cannot keep up with the pace of the deformation amount of silicon and plastic deformation (crack) occurs.
The thickness of the oxide film on the surface which is in the film vibration mode at the time of dot formation similar to that of a completely bare wafer was found to be 1,500 to 2,000 angstroms by experiments. When the oxide film on the surface has the thickness of about 1,500 angstroms or less, dots can be formed in the film vibration mode similar to that of a bare wafer.
Preferably, in addition to the marking parameters, a parameter of disposing the beam profile converting means 5 at either the front or post stage of the liquid crystal mask 4 is included. The beam profile converting means S takes the form of a dot matrix of the same size as that of the pixel matrix of the liquid crystal mask 4 and converts the energy density distribution of a laser beam into a required distribution. The beam profile converting means adjusts the thermal distribution in the irradiation pattern dots, thereby adjusting the height of the protrusion of the dot mark.
The maximum length of each of pixels in the liquid crystal mask 4 is specified to 50 to 2,000 xcexcm due to the limit of the resolution of an existing lens system when a laser beam transmitted the liquid crystal mask 4 is condensed on the article to be marked so that the maximum length of one dot is set to 1.0 to 15.0 xcexcm by a lens system. When the maximum length (diameter) of one dot is smaller than 1.0 xcexcm, it is difficult to read each dot by an existing optical system sensor. When the maximum length exceeds 15.0 xcexcm, not only a sufficient amount of information cannot be marked on a predetermined area but also the marking area is limited. Each of the values is 3/20 to 1/100 of 100 xcexcm which is the maximum size of the dot mark permitted by the present SEMI standard. It can be understood how small the sizes are.